1. Field of the Invention
The present invention relates to the field of computer component assembly and in particular to an assembly of a die to a heat conductor.
2. Discussion of Related Art
In the design and manufacture of computer hardware, meeting certain thermal requirements can be essential. In particular a silicon microchip (die) placed into a circuit package, can have a requirement to remove heat generated by the microchip during operation. The circuit package may have a barrier of plastic covering the die. In the case of laptop computers, a heatpipe acting as a heat conductor may be attached to the circuit package containing the die to help carry off the heat. As illustrated in FIGS. 1a and 1b, the heatpipe 116 can have a metal part called a spreader plate 104 that is placed between the circuit package 110 and the heatpipe 116 to thermally mate the smaller heatpipe contact area to the circuit package 110. The heatpipe 116, circuit package, 110, and spreader plate 104 are clamped to a printed circuit board substrate (substrate) 112 using several fasteners 102. The clamping process can places unequal forces (loads) F1 and F2 (only two fasteners are shown for clarity but four or more fasteners may be used) on the heatpipe 116 and spreader plate 104. There can potentially be as many unequal forces applied as there are fasteners 102. As a result, some of the computer components (heatpipe 116, spreader plate 104, circuit package 110) may flex and/or shift. With movement of the computer components 116, 104, 110, thermal interface materials 108, 114 placed between the heatpipe 116 and the spreader plate 104 and between the spreader plate 104 and the circuit package 110 may take on a varying thickness. A varied thickness in the thermal interface materials 108, 114 as well as an increase in thermal interface material 108, 114 thickness will both increase thermal resistance.
Illustrated in FIGS. 1a, 1b, and 1c is an apparatus to provide the clamping force using fasteners 102 such as screws or bolts. These fasteners 102 connect the spreader plate 104 to the substrate 112 with the circuit package 110 in between. Each fastener 102 applies a force (F1, F2) that contributes to the total clamping force (F1+F2). The spreader plate 104 and the substrate 112 place in compression a first thermal interface material (TIM 1) 108, the circuit package 110, and a second thermal interface material (TIM 2) 114. Even small differences in the dimensions of the spreader plate 104 or the circuit package 110 or in the torque applied 113 to each fastener 102 is sufficient to cause one fastener 102 to have a force different F1xe2x89xa0F2 from the other fasteners 102. As a result, the spreader plate 104 may tilt (FIG. 1b) and the thermal interface materials 108, 114 can each take on a varying thickness. In addition, if the forces (F1, F2) applied are too great for the spreader plate 104 stiffness, the spreader plate 104 may bow (FIGS. 1a and 1c). If the spreader plate 104 is sufficiently stiff, the spreader plate 104 may tilt as a result of the unequal forces F1, F2 (FIG. 1b). Both bending and tilting of the spreader plate 104 are simultaneously possible with the result that TIM 1108 can flow in response thereby creating a non-uniform TIM 1108 thickness. The second thermal interface material (TIM 2) 114, positioned between the spreader plate 104 and the heatpipe 116, is outside the clamping force (F1+F2) but can still flow in response to the movement of the spreader plate 104, with the result of a non-uniform TIM 2114 thickness. The consequence of non-uniform TIM 108, 114 thicknesses is reduced performance because of a local and/or overall temperature increase in the circuit package 110.
Additionally, in response to these unequal loads (F1, F2), TIM 1108 and TIM 2114 may develop voids, and TIM 1108 and TIM 2114 may separate from the spreader plate 104 and/or the heatpipe 116. As a result, an increase in the thermal resistance recurred by TIM 1108 and TIM 2114 on to thickness differences and voids/separations can occur.
Connecting the spreader plate 104 to the heatpipe 116 may be accomplished without a thermal interface material by using a close fit of the components such as an interference fit that requires tight dimensional tolerances between mating surfaces. To minimize thermal resistance, close direct contact is required to avoid air gaps between the two mating parts. Alternatively, the connection may be accomplished with the thermal interface material between the heatpipe 116 and the spreader plate 104. The thermal interface material (TIM) should also be thermally conductive and may be a grease, a solder, selected from a range of adhesives, or other materials. The interface dimensions, the thermal interface material, and a method of holding the computer components in a stacked position (stack), are important.
When an adhesive or solder is used as the thermal interface material, and bond strength is required, proper assembly force is necessary to ensure good bond strength. If no bond strength is required, a thermal interface material may be used that does not set up as do the adhesives and solders. However, regardless of whether a TIM sets up like an adhesive or does not set up such as with a grease, during the time the TIM can flow or deform requires the thickness to be controlled as well as the creation of gaps and voids to be minimized. Such voids may exist within the TIM and gaps can exist at the TIM surfaces. Additionally, the thickness of the material may be applied unevenly. As a result, heat conduction through the material and interface surfaces will be less efficient.